1. Field of the Invention
The present invention relates to fiber reinforced thermoplastic resin molded products having excellent mechanical properties and a good surface appearance, fiber reinforced thermoplastic resin pellets and the methods of producing such pellets useful for producing such products. The present invention further relates to the methods of producing the molded product having a good surface appearance, using the pellet.
The molded product of the present invention can be used in various fields as follows: motorcar interiors such as a console box, an instrument panel and a trim; motorcar exteriors such as a bumper, a fender, a front grill, a rear spoiler and a side protector; motorcar-related members such as members inside an engine room, for example, a fan, a fan shroud, a battery tray and a fuse box; housings for electrical equipment such as a notebook-sized personal computer and a portable telephone; structural members of electric appliances; office furniture; building materials; and containers.
2. Description of Related Art
A fiber reinforced thermoplastic resin in which its matrix is a thermoplastic polymer and its reinforcing fibers are glass fibers has widely been used, exclusively in a pellet form, as a material for injection molding making mass production possible. As pellets containing glass fibers, there are known a short fiber reinforced thermoplastics pellet (a compound pellet) obtained by melting and compounding chopped glass fibers having a length of 3-12 mm and a matrix thermoplastics polymer in an extruder, extruding the blend into a strand form, and cutting the resultant, and a long fiber reinforced thermoplastics pellet obtained by causing continuos glass fibers tow to pass through a melted matrix thermoplastics polymer bath to impregnate the tow with the melted resin, pulling out and cooling the tow into a strand form and then cutting the strand.
Concerning the short fiber reinforced thermoplastics pellet, its glass fibers are damaged in the melt compounding step, so that the length of the fibers in the actually-obtained pellet becomes far shorter than that of the fibers before the compounding. The fibers are also damaged in injection molding. Thus, the weight average fiber length (Lw) of an injection-molded product using such a short fiber reinforced thermoplastics pellet is reduced to about 0.3-0.6 mm. The glass fibers, therefore, do not become entangled with each other so that reinforcing effect is not exhibited, resulting in poor mechanical properties.
In the long fiber reinforced thermoplastics pellet, since its fibers are not damaged during the production thereof, which is different from the short fiber reinforced thermoplastics pellet, an injection-molded product having satisfactory mechanical properties can be obtained. However, the length of the fibers causes the dispersibility of the fibers to deteriorate, resulting in such appearance badness that the fibers bundle is in sight from the surface of the product.
In particular, when a method is used where a master pellet having high concentration of its glass fibers is mixed with a pellet containing only a matrix thermoplastics polymer and does not contain any glass fiber at the time of molding so as to make up the contained amount of the glass fibers to a given value, causing a problem where appearance badness resulting from non-uniform dispersion of the glass fibers comes into prominence.
Thus, investigations have been made for dispersing glass fibers uniformly by setting up an especial blending nozzle onto a molding machine, making a gate of a mold narrow, or setting the backpressure of a screw at the time of molding to a high value. However, such manners for enlarging shearing force in blending and injection steps are not very effective for an improvement in the dispersibility of the fibers. Conversely, the fibers are unfavorably damaged.
From the viewpoint that it is preferable for an improvement in dispersibility of glass fibers to get the glass fibers in the state of a monofilament sufficiently wet with a matrix thermoplastics polymer, a method of getting the glass fibers wet with the matrix thermoplastics polymer in the process of producing a pellet is disclosed (Japanese Patent Application Laid-Open No. 3-13305). However, the improvements in the dispersibility of the fiber and a surface appearance are unsatisfactory.
Japanese Patent Application Laid-Open No. 5-239286 discloses a method of specifying the MI (melt index) of a matrix thermoplastics polymer and the blend ratio of the resin to glass fibers to raise dispersibility of the fiber and reduce the damage of the fibers. Japanese Patent Application Laid-Open No. 5-124036 discloses a method of using a resin having a lower melting viscosity as a master pellet of a high glass fiber content, and using a resin having a higher melting viscosity as a pellet not containing any glass fiber, to raise dispersibility of the glass fibers and mechanical properties of a molded product.
In these methods in the prior art, the surface appearance of a molded product can be improved to some degree, and there are not brought such bad results that the glass fibers are projected from the surface of the product. However, the dispersibility of the glass fibers is not completely improved, so that the glass fibers may be present in a bundle form near the surface of the product. If the product includes pigments and dyes and the glass fibers tow is present near its surface, the concentration of the colored resin is small at the area where the tow is present. Thus, the color becomes uneven so as to result in appearance badness. In other words, even when the glass fiber is not in sight from the surface, appearance badness such as color unevenness cannot be overcome if the glass fibers are non-uniformly dispersed. Furthermore, if the glass fibers are non-uniformly dispersed and the glass fibers tow is locally present, physical properties such as mechanical properties of the product are aversely influenced. Accordingly, there remain problems that physical properties vary between lots of injection-molded products or between locations of the same molded product.